BEIRUT (Reuters) - A Syrian rebel alliance agreed to a three-day country-wide ceasefire announced by the Syrian army on Wednesday and, although fighting and air attacks continued, U.S. Secretary of State John Kerry expressed hope a more significant truce could be achieved.

The truce is the first to be declared across the whole country since the one brokered by foreign powers in February to facilitate talks to end the five-year-old civil war. This has mostly unraveled.

Wednesday's ceasefire covers the three-day Eid al-Fitr holiday celebrated by Muslims to mark the end of the fasting month of Ramadan.

But a rebel group and a war monitor said little had actually changed on the ground.

Syria's military high command said in a statement that "a regime of calm will be implemented across all territory of the Syrian Arab Republic for a period of 72 hours from 1 a.m. on July 6 until 2400 on July 8, 2016".

The Syrian government uses the term "regime of calm" to denote a temporary ceasefire.

The Free Syrian Army rebel alliance later said it would respect an Eid holiday ceasefire, but only if government forces also abided by it.

"We, the armed revolutionary groups in Syria, welcome any effort towards a ceasefire for the happy Eid al-Fitr period. We declare we will abide by it so long as the other side does the same," said the statement, posted on the Twitter account of Mohammed Alloush, the former chief peace negotiator for Syria's mainstream opposition.

"Until now, (the government) has not abided by what it has announced, in that it has launched a number of attacks in various areas today," the statement said.

Alloush is also the representative of the powerful Jaish al Islam rebel faction in the opposition High Negotiations Committee (HNC).

The statement said the rebel alliance welcomed international efforts that had led to the announcement from the government of Syrian President Bashar al-Assad, but that attacks had not ceased as a result.

Jaish al Islam spokesman Islam Alloush said: "The regime has made this announcement purely to escape international pressure. On the ground, I don't think anything has changed."

Jaish al Islam said in a separate statement that, despite the announced truce, government and allied forces had attacked the town of Maydaa, in the Eastern Ghouta area east of Damascus. Maydaa has been held by Jaish al Islam.

The Syrian Observatory for Human Rights said on Wednesday that government and allied forces had taken almost complete control of Maydaa and that fighting continued. Syrian state media said the army and its allies had taken ground from "terrorists" in the area. The Syrian government describes all groups fighting against it as terrorists.

The Britain-based Observatory, which monitors the Syrian conflict, also said there had been rebel and government shelling in areas around the northern Syrian city of Aleppo, and air strikes had hit towns in the northern Aleppo countryside on Wednesday.

Syrian state media also reported army operations against Islamic State militants across the country on Wednesday.

U.S. Secretary of State John Kerry welcomed the Syrian army's announcement, adding that discussions were under way to try to extend the truce.

"We are trying very hard to grow these current discussions into a longer-lasting ... enforceable, accountable cessation of hostilities that could change the dynamics on the ground," Kerry told a news conference in Tbilisi, Georgia.

The open-ended February "cessation of hostilities" truce, which was intended to facilitate talks to end the five-year-old civil war, was agreed with many opposition militias, but did not include the al-Qaeda affiliated Nusra Front or Islamic State.

But the truce has mostly collapsed since then and the Syrian army and the Russian military, which supports Syrian President Bashar al-Assad, have announced a number of temporary local truces in areas of intense fighting, for example in the city of Aleppo or near the capital Damascus.

But air strikes and fighting have often continued in spite of the declarations.

(Reporting by Lisa Barrington in Beirut, additional reporting by by Lesley Wroughton and Margarita Antidze in Tbilisi and Omar Fahmy in Cairo.; Editing by Toby Chopra)